The present invention relates in general to a carrier wave recovery circuit and, more particularly, to a circuit wherein a carrier wave is reproduced by means of a digital synthesizer for use in the receiving portion of an image information transmitting apparatus such as a facsimile system.
With the increased emphasis on an automated office and the increased use and popularity of electronic apparatus in the office environment, a reliable and high-quality information transmitting apparatus, such as a facsimile system, is of greater importance. In a prior art information transmitting apparatus, vestigial sideband (VSB) transmission was utilized wherein a part of the carrier frequencies is suppressed and a signal synchronized in frequency and phase with the carrier wave is recovered and reproduced in a demodulator utilizing a phase-locked-loop (PLL) technique.
A typical example of a prior art PLL circuit of this type is shown in FIG. 1. A phase comparator 2 operates as a multiplier for two AC input signals f.sub.s and f.sub.c. The output of the phase comparator 2 is applied through a low-pass filter 2 to an amplifier 3 which outputs a voltage Vd proportional to the time variation of a phase difference (.theta..sub.s -.theta..sub.c) detected. A voltage-controlled oscillator 4 (hereinafter referred to as "a VCO" when applicable), which is controlled by the input voltage Vd, operates to cause the signal f.sub.c to approach the signal f.sub.s. The system is stabilized when f.sub.s =f.sub.c.
In prior art information transmitting apparatus utilizing such an analog PLL technique, various problems are encountered including the use of a circuit that is susceptible to noise, a circuit that requires a great number of adjustments, and a circuit whose circuit having parameters susceptible to variations and changes after a long period of usage.
In order to eliminate these difficulties, a PLL circuit using a digital phase comparator has been proposed in the art. However, that circuit also suffers from the problem that, as the output frequency is changed according to the input voltage applied to the VCO, the output frequency is varied by supply voltage variations and by external noise.
Accordingly, objects of the invention are to provide a carrier reproduction system in which the VCO is eliminated, carrier reproduction is performed completely in a digital mode thereby to eliminate all of the above-described drawbacks accompanying a conventional carrier reproduction system, and to provide a carrier reproduction system which has excellent stability and reliability, is capable high productivity during manufacture and is suitable for use in a video data transmission system such as a facsimile system.